Motors and pumps operating upon the principles invented by R. J. L. Moineau, for example U.S. Pat. No. 1,892,217 of Dec. 27, 1932, and Clifford Allen, for example, U.S. Pat. Nos. 3,512,904 of May 19, 1970 and 3,938,744 of Feb. 17, 1976, are now quite well known. In all cases, these involve a fluid motor composed of a helical gear pair wherein an inner member has one or more exterior helical threads and a cooperating outer member has one or more internal helical threads, the number of helical threads on the inner and outer members differing by one. Moineau motors have been used nearly 20 years for drilling oil, gas and water wells in which the motor is placed down-hole at the bottom of the drill-string of pipe, the purpose being to have the power near the work being done as well as to save wear and tear on the drill stem, which does not necessarily need to be rotated, and also to allow the motor to be directed for the purpose of changing the path of the bore-hole in a manner known as " directional drilling". However, the life of the motor has been inherently greatly shortened by the fact that such a motor generally has a resilient outer member in contact with the hydraulic force of the fluid pumped down from the surface, producing the power output to cut the hole. To prevent fast wear of the resilient outer member, or in fact destruction in a matter of minutes, it is necessary to control the pressure drop of the fluid through the motor so that it does not become excessive.
The present art in making elastomeric compounds for outer member molding limits the pressure drop in the common drilling motor sizes on the order of 300 pounds per square inch of fluid pressure. Accordingly, during operation the operator endeavors to keep up to this pressure in order to drill at optimum penetration rate, as time savings are economically most important. Heretofore, the operator has had no way to accurately know the pressure drop of the fluid through the motor, so the operator has held the drill off bottom, then set it down on bottom and noted the pressure rise on his pump output showing the input down the drillpipe, allowing it to rise, perhaps, 300 pounds per square inch above what it was when he checked it with the drill just off bottom. As most of the pressure is used to circulate the fluid down to the bottom and return, to keep the cuttings removed, to keep the walls of the bore hole conditioned to prevent loss in the circulation as well as preventing blowouts from formation pressure; and, as the required pressure is escalating with every downward movement in penetration, the operator must constantly revalue and adjust to be sure he is not destroying the motor and yet that he is getting maximum work therefrom. As can be expected, the tendency has been to prematurely wear out the motor at the resulting increased expense and thus to restrict its use to special situations such as directional drilling, with return to conventional drilling without the motor as soon as possible because of the lack of a control system for its operation.